Ewing's sarcoma (EWS) cells contain significantly higher levels of poly(ADP-ribose) polymerase (PARP) mRNA, protein and enzymatic activities than any other eukaryotic cells. Evidence from our laboratory showed that increased transcription, rather than mRNA stability, contributes to the elevated PARP levels. It has been proposed that alterations in the normal turnover rate of PARP may also contribute to the total cellular PARP content as well as to the apoptotic response of Ewing's sarcoma cells to ionizing radiation. To address this possibility, we compared the turnover of PARP in EWS cells (A4573), which contain high PARP and are relatively radiosensitive, with that in laryngeal squamous cell carcinoma cells (SQ-20B), which have low PARP and are radioresistant. Results showed that PARP turnover parameters are nearly identical in both cells types. These data conclusively demonstrate that PARP turnover is not a determinant of either the elevated PARP content or the radiation response of EWS cells.